Operating Expense Ratio (OER): What It Is, the Formula and How to Calculate It
By Briant CarcamoCEO & Founder of Vizibly 14 min read
Calculating an operating expense ratio takes one division. Explaining why it moved is the actual job.
The short version: OER tells you how much of every dollar the property collects goes to running it, and it moves when either side of the fraction moves.
Quick Answer The operating expense ratio (OER) is the share of a property's income that goes to operating expenses. The formula is OER = Operating Expenses ÷ Effective Gross Income × 100. A multifamily property with $1,031,668 of operating expenses and $2,055,600 of effective gross income has an OER of about 50.2%. Debt service, capital expenditures, depreciation and income taxes are left out.
What Is the Operating Expense Ratio (OER)?
The operating expense ratio measures how much of a property's income it costs to run the property. It's a percentage, usually calculated for a full year.
Read it as cents on the dollar. An OER of 50% means 50 cents of every dollar the property collects goes to payroll, repairs, utilities, insurance, taxes and the rest of the operating budget. The other 50 cents is net operating income (NOI).
That's why owners check it right after NOI. NOI tells them how much the property earned. OER tells them how efficiently it earned it, in a form they can compare across properties of different sizes.
One note on the name. "Expense ratio" also describes what a mutual fund charges, and "OER" is also short for owners' equivalent rent in the Consumer Price Index. This article is about the property version.
Which Income Goes in the Denominator?
The top of the fraction is always operating expenses. The bottom is where definitions drift.
A.CRE's real estate glossary defines OER as "The ratio of Operating Expenses to Effective Gross Revenue." Wall Street Prep's OER guide divides total operating expenses by gross operating income, which it builds by deducting vacancy and credit loss from potential gross income.
Different labels, same idea: income after vacancy and collection losses, plus other income. In multifamily, that's effective gross income (EGI), and it's the denominator this article uses.
Some reports divide by gross potential rent (GPR) instead. That gives a lower ratio for the same property, so check the denominator before you compare two OERs. The example below shows how much it matters.
The Operating Expense Ratio Formula
Here's the formula:
Operating Expense Ratio = Operating Expenses ÷ Effective Gross Income × 100
NOI is what's left of EGI after operating expenses. So OER and NOI margin (NOI as a share of EGI) always add up to 100%:
OER + NOI Margin = 100%
A 50% OER is a 50% NOI margin. Push one down and the other goes up by the same amount.
What Counts as an Operating Expense
Operating expenses are the recurring costs of running the property:
- Payroll and benefits for site staff
- Repairs and maintenance (R&M)
- Make-ready and turnover
- Contract services (landscaping, pest control, trash)
- Utilities the property pays
- Marketing
- Administrative costs (software, supplies, legal and professional fees)
- The management fee
- Property insurance
- Property taxes
What Stays Out of the Ratio
Four things never belong in the numerator:
- Debt service. Mortgage principal and interest are financing costs. Put them in and OER starts measuring the loan instead of the operation.
- Capital expenditures. Roofs, HVAC replacements and unit renovations are long-lived improvements, not this year's cost of running the property.
- Depreciation and amortization. These are accounting entries, not cash spent on operations.
- Income taxes. They depend on the owner's tax situation. Property taxes stay in. Income taxes stay out.
Replacement reserves are the gray area. Some owner and lender formats show them above NOI, and others show them below. Match the format your owner uses and keep it the same from year to year.
How to Calculate the Operating Expense Ratio: A Multifamily Example
Here's an illustrative 120-unit property with an average market rent of $1,500 a month. The numbers are round on purpose.
- Start with effective gross income. Gross potential rent of $2,160,000, minus $212,400 of vacancy, loss to lease, concessions, bad debt and one model unit, plus $108,000 of other income = $2,055,600.
- Total the operating expenses. The ten lines below add up to $1,031,668.
- Divide. $1,031,668 ÷ $2,055,600 = 0.502.
- Multiply by 100. The OER is 50.2%.
- Check the other side. NOI is $2,055,600 − $1,031,668 = $1,023,932, a 49.8% NOI margin. 50.2% + 49.8% = 100%.
| Expense line | Annual amount | % of EGI | Per unit |
|---|---|---|---|
| Payroll | $280,000 | 13.6% | $2,333 |
| Repairs and maintenance | $70,000 | 3.4% | $583 |
| Make-ready and turnover | $50,000 | 2.4% | $417 |
| Contract services | $45,000 | 2.2% | $375 |
| Utilities | $150,000 | 7.3% | $1,250 |
| Marketing | $25,000 | 1.2% | $208 |
| Administrative | $30,000 | 1.5% | $250 |
| Management fee (3% of EGI) | $61,668 | 3.0% | $514 |
| Insurance | $90,000 | 4.4% | $750 |
| Property taxes | $230,000 | 11.2% | $1,917 |
| Total operating expenses | $1,031,668 | 50.2% | $8,597 |
Illustrative example. Not a benchmark for any market.
The % of EGI column does the real work. Each line's ratio shows where the 50.2% comes from. Here, payroll and property taxes make up roughly half of it, and utilities and insurance add another 11.7 points.
When an owner asks why OER moved, this is the table you answer from.
Same Property, Different Denominator
Divide the same expenses by gross potential rent instead:
$1,031,668 ÷ $2,160,000 = 47.8%
Nothing about the property changed. The ratio dropped 2.4 points because the denominator got bigger.
If a regional report shows one property at 47.8% and another at 50.2%, confirm they're measured the same way before anyone asks the second site what went wrong.
Annual, T12 or One Month?
Calculate OER on a full year: the annual budget, or the trailing 12 months (T12) of actuals.
A single month's OER is a mood, not a trend. A summer turn wave, an annual contract invoice or a property tax true-up can swing one month's ratio without saying anything about the year.
Operating Expense Ratio vs NOI Margin and Expense per Unit
OER is one of several ways to look at the same operating statement. Each answers a different question:
| Metric | Formula | What it tells you |
|---|---|---|
| Operating expense ratio | Operating expenses ÷ EGI | The share of income spent running the property |
| NOI margin | NOI ÷ EGI | The share of income left after running the property (100% − OER) |
| Operating expenses per unit | Operating expenses ÷ number of units | What it costs to run each unit, whatever the rent level |
| Fund expense ratio | Annual fund costs ÷ fund assets | What a mutual fund or ETF charges investors. Unrelated to property operations |
OER and expense per unit work best together. A property with lower rents can run lean on cost per unit and still post a high OER, because the same costs sit on a smaller income base.
Take the example property's $1,031,668 of expenses and put it against 20% less income. OER jumps from 50.2% to about 62.7%. Same costs per unit, different story.
NOI margin is the version owners translate into value, because value is roughly NOI divided by a cap rate. We cover cap rates and property value in a separate post.
What Is a Good Operating Expense Ratio for Multifamily?
There's no single good number. There are reference points, and there's your own property's history.
Start with the national picture. NAA's 2025 Dollar of Rent analysis found that day-to-day operating costs "such as insurance, utilities, maintenance and management fees" took 27 cents of every rent dollar in 2025. Property taxes took another 10 cents and payroll 7 cents.
Add those three and roughly 44 cents of each rent dollar went to running the property. That's an operating expense ratio in the mid-40s across the 13,471 communities in NAA's sample, measured against rental revenue.
Our multifamily financial fundamentals post puts the working range higher, at roughly 50–55% of EGI.
The two reference points don't match, and that's the useful part. A national average blends new and old, large and small, high-tax and low-tax properties. Your property is one data point inside it. Here's what pushes a property's OER up or down:
- Who pays utilities. A property that pays water, sewer and trash carries those costs in its expenses. One that bills them back to residents shows the reimbursements as other income. Utility billing, including RUBS, is regulated at the state and local level, so check your local rules.
- Property tax burden. NAA's 2024 rent dollar breakdown found that in high-tax states such as Texas, New Jersey and Illinois, property taxes "can rise to 13 cents or more" of each rent dollar, against 10 cents nationally.
- Age and condition. Older buildings carry more repair and maintenance cost.
- Size. On a small property, payroll and fixed contracts are spread across fewer units.
- Rent level. The same costs on lower rents produce a higher ratio.
- Lease-up. A property in lease-up runs thin income with a full staff, so its OER stays high until occupancy catches up.
So compare your OER to its own history, to its budget and to similar properties in the same market. Then use the result to ask better questions:
OER above history or comps: Check the revenue side first (occupancy, concessions, bad debt). Then check the lines set off-site (taxes, insurance, utility rates). Then check the lines the team controls.
OER below history or comps: Check whether repairs were coded as capex, whether expenses landed in the right month, and whether maintenance is being deferred. Owners and lenders notice a ratio that looks too good.
A lower OER isn't automatically a better one. A ratio that's low because the paint and carpet budget was pushed into next year will come back with interest.
Operating Expense Ratio Has Two Moving Parts
OER is a fraction, so it moves when either part moves. Site teams tend to think of it as an expense number. Half the time, it's a revenue number.
Take the example property and change one thing at a time:
Vacancy up 2 points: $1,030,372 ÷ $2,012,400 = 51.2% Insurance up 15%: $1,045,168 ÷ $2,055,600 = 50.8%
In the first case, two extra points of vacancy cut EGI by $43,200. Expenses actually fell by $1,296, because the management fee is 3% of EGI. OER still rose a full point.
In the second case, a $13,500 insurance increase moved OER by about 0.7 points. Two points of vacancy moved it more than a 15% insurance renewal did.
The same squeeze shows up in NAA's national numbers. Among the same communities, revenue grew 2.7% in 2025 while operating expenses grew 4.6%. Run the ratio on NAA's per-unit figures and you can watch it climb:
2024: $7,848 ÷ ($1,504 × 12) = 43.5% 2025: $8,213 ÷ ($1,545 × 12) = 44.3%
Our arithmetic on NAA's published per-unit revenue and expense figures.
Here's what the site team controls on each side of the fraction.
On the revenue side:
- Occupancy and turn speed. On the example property, one point of vacancy is $21,600 of income and adds about half a point to OER.
- Concessions and bad debt. Both come straight off EGI.
- Other income. Fees, parking and utility reimbursements raise EGI. Fees and utility billing are regulated locally, so check the rules before you change what you charge.
On the expense side:
- Turn costs and R&M. Code invoices to the unit, so you can see which turns cost the most.
- Contract services. Rebid them on a schedule instead of letting them roll over.
- Utilities. Leak response and common-area usage are on-site decisions, even when the rate isn't.
- Coding. Put each invoice on the right line, in the right month.
Some lines mostly aren't yours to control. NAA puts it plainly: "Tax assessments, insurance premiums and utility rates are set elsewhere." You can't negotiate the tax bill on-site, but you can explain it.
How the Operating Expense Ratio Shows Up in Budget Review
In a budget review, OER usually comes right after NOI. The owner looks at the budgeted ratio, compares it to last year and asks why it moved.
A good answer separates the two sides of the fraction. Here's the example property's illustrative budget for next year:
| Line | Last year | Budget | Change |
|---|---|---|---|
| Effective gross income | $2,055,600 | $2,111,100 | +$55,500 (+2.7%) |
| Payroll | $280,000 | $291,200 | +$11,200 |
| Repairs and maintenance | $70,000 | $72,000 | +$2,000 |
| Make-ready and turnover | $50,000 | $51,500 | +$1,500 |
| Contract services | $45,000 | $46,500 | +$1,500 |
| Utilities | $150,000 | $162,000 | +$12,000 |
| Marketing | $25,000 | $25,000 | $0 |
| Administrative | $30,000 | $31,000 | +$1,000 |
| Management fee (3% of EGI) | $61,668 | $63,333 | +$1,665 |
| Insurance | $90,000 | $103,500 | +$13,500 |
| Property taxes | $230,000 | $243,800 | +$13,800 |
| Total operating expenses | $1,031,668 | $1,089,833 | +$58,165 (+5.6%) |
| Operating expense ratio | 50.2% | 51.6% | +1.4 points |
Illustrative example.
Here's how to explain it in one breath:
OER goes from 50.2% to 51.6%. Income is up 2.7% and expenses are up 5.6%. Insurance, property taxes and utilities account for $39,300 of the $58,165 increase. Payroll adds $11,200 on a 4% wage assumption. The lines the team manages day to day (R&M, make-ready, contract services, marketing and admin) are up $6,000 combined.
That's an answer an owner can act on. "Expenses went up" isn't.
When the reasoning behind a line isn't on the page, owners start discounting the whole budget. That's the operator–owner trust gap, and an unexplained jump in OER is one of the fastest ways to open it.
This is where the review process matters as much as the math. Vizibly keeps reviewer comments and decisions on the budget lines themselves, so the reason the insurance line jumped is sitting next to the number when the owner opens the budget.
The conversation doesn't end at approval. Track OER against budget during the year on a T12 basis. When the insurance renewal or a tax reassessment comes in above budget, the rest of the year's ratio is already different. Reforecasting as the year unfolds keeps the number the owner sees current.
Common Operating Expense Ratio Mistakes
- Comparing ratios with different denominators. On the example property, EGI vs GPR is a 2.4-point difference with nothing else changed.
- Putting debt service or capex in the numerator. A $60,000 roof replacement coded to repairs adds 2.9 points to this property's OER.
- Comparing a bill-back property to an all-utilities-paid one. The same building can show very different ratios depending on who pays the water bill.
- Judging a lease-up against stabilized comps. Thin income and a full staff make OER look high until occupancy catches up.
- Reading one month. Use the annual budget or a T12.
- Deferring maintenance to hit a ratio. It lowers OER this year and shows up as a bigger repair or capital project later.
FAQ
What is a good operating expense ratio for multifamily?
There's no single number: NAA's 2025 data puts operating costs, property taxes and payroll at roughly 44 cents of each rent dollar nationally, and our working range is 50–55% of EGI. The most useful comparison is your property's own history and similar properties in the same market.
Do you calculate OER on gross income or effective gross income?
Most real estate definitions use effective gross income, sometimes called gross operating income or effective gross revenue, which is income after vacancy and collection losses. Dividing by gross potential rent gives a lower ratio, so state which one you use.
Does the operating expense ratio include property taxes?
Yes, property taxes are an operating expense, and NAA's 2025 analysis calls them the single largest operating expense nationally. Income taxes are excluded.
Does OER include debt service or capital expenditures?
No. Mortgage payments, capital expenditures, depreciation and income taxes all sit below the NOI line, so they stay out of the ratio.
How is the operating expense ratio related to NOI?
OER and NOI margin always add up to 100% of effective gross income, so a 50.2% OER means 49.8% of EGI becomes NOI.
What is the 50% rule in real estate?
The 50% rule is an investor shortcut that assumes operating expenses, excluding the mortgage, will take about half of gross rent. It's fine for a quick screen, but a property budget should be built line by line from actuals.
The Punchline
OER is one division problem: operating expenses over effective gross income.
It moves when income moves and when costs move. Know which one moved, line by line, and the owner conversation gets a lot shorter.
Same denominator. Full year. Line-level ratios. Revenue side first.
Want your site team reading expense ratios the way owners do? Financials 101 for Property Managers walks through the whole P&L, with an operating expense ratio calculator built in.
Sources
- National Apartment Association, "Where a Dollar of Rent Went in 2025," Leah Cuffy and Michael Liu, October 14, 2026. https://naahq.org/news/where-dollar-rent-went-2025
- National Apartment Association, "Where Does a Dollar of Rent Go?," Leah Cuffy, February 24, 2026. https://naahq.org/news/where-does-dollar-rent-go
- A.CRE (Adventures in CRE), "Operating Expense Ratio" (glossary), updated July 2025. https://www.adventuresincre.com/glossary/operating-expense-ratio/
- Wall Street Prep, "Operating Expense Ratio (OER): Formula + Calculator," updated June 2024. https://www.wallstreetprep.com/knowledge/operating-expense-ratio-oer/
